Determining the low level oxygen demand of combustible materials in aqueous dispersions

ABSTRACT

A METHOD FOR MEASURING OXYGEN DEMANDS OF AQUEOUS DISPERSIONS CONTAINING COMBUSTIBLE MATERIAL IN THE PARTS PER BILLION RANGE. THE METHOD BROADLY COMPRISES THE STEPS OF FLOWING A FEED GAS STREAM COMPOSED OF AN INERT GAS CONTAINING A SMALL AMOUNT OF OXYGEN INTO A CONFINED, HEATED COMBUSTION ZONE, AT A CONSTANT RATE. THE FEED GAS STREAM IS PASSED THROUGH A COMBUSTION SUPPORTING, POROUS CATALYST BED CONTAINED WITH THE COMBUSTION ZONE, WHICH IS HEATED TO A COMSUTION SUPPORTING TEMPERATURE. THE GAS STREAM IS THEN FED INTO A DETECTOR FOR MEASURING SMALL AMOUNTS OF FREE OXYGEN. A SMALL ALIQUOT OF THE AQUEOUS DISPERSION TO BE ANALYZED IS INJECTED INTO THE FEED GAS STREAM WITHIN THE COMBUSTION ZONE UPSTREAM FROM THE CATALYST BED. THE RESULTING GASEOUS PRODUCTS ARE SWEPT ALONG IN THE GAS STREAM FROM THE COMBUSTION ZONE INTO AN OXYGEN DETECTOR. THE OXYGEN DEPLETION IN THE GAS STREAM IS CALIBRATED TO YIELD A MEASUREMENT OF THE OXYGEN DEMAND OF THE AQUEOUS DISPERSION. TO ACHIEVE ACCURATE MEASUREMENTS AT LOW LEVELS OF OXYGEN DEMAND, THE METHOD IS OPERATED WITH THE FEED GAS STREAM CONTAINING OXYGEN WITHIN THE RANGE FROM ABOUT 0.5 UP TO ABOUT 10 PARTS PER MILLION BY VOLUME. A CATALYST BED AT LEAST 8 CENTIMETERS LONG ASSURES COMPLETE AND REPRODUCIBLE COMBUSTION. IT IS ALSO ESSENTIAL THAT THE AQUEOUS SAMPLE TO BE ANALYZED BE SUBSTANTIALLY FREE OF DISSOLVED OXYGEN UPON INJECTION INTO THE COMBUSTION ZONE.

July 25, 1972 J. L. TEAL r-rrAL 3,679,364

DETERMINING THE LQW LEVEL OXYGEN DEMAND OF COMBUSTIBLE MATERIALS IN AQUEOUS DISPERSIONS Filed Oct. 30, 1970 I I I; I I I I INVENTOR5. James L. 7220/ Ch ar/es 1 Hamf/fon k Dennis 7. C/l'ff'or HTTORNEY United States Patent US. Cl. 23-230 PC 5 Claims ABSTRACT OF THE DISCLOSURE A method for measuring oxygen demands of aqueous dispersions containing combustible material in the parts per billion range. The method broadly comprises the steps of flowing a feed gas stream composed of an inert gas containing a small amount of oxygen into a confined, heated combustion zone, at a constant rate. The feed gas stream is passed through a combustion supporting, porous catalyst bed contained within the combustion zone, which is heated to a combustion supporting temperature. The gas stream is then fed into a detector for measuring small amounts of free oxygen. A small aliquot of the aqueous dispersion to be analyzed is injected into the feed gas stream within the combustion zone upstream from the catalyst bed. The resulting gaseous products are swept along in the gas stream from the combustion zone into an oxygen detector. The oxygen depletion in the gas stream is calibrated to yield a measurement of the oxygen demand of the aqueous dispersion.

To achieve accurate measurements at low levels of oxygen demand, the method is operated with the feed gas stream containing oxygen within the range from about 0.5 up to about parts per million by volume. A catalyst bed at least 8 centimeters long assures complete and reproducible combustion. It is also essential that the aqueous sample to be analyzed be substantially free of dissolved oxygen upon injection into the combustion zone.

INVENTION BACKGROUND In a co-pending application, Ser. No. 452,809, filed May 3, 1965, now US. Pat. -No. 3,560,156, of which the instant application is a continuation-in-part, applicants have described a method for determining the total oxygen demand of aqueous systems in a gas stream containing from about 10 up to 10,000 parts per million by volume of oxygen. Combustion is effected in a catalyst bed at a combustion supporting temperature within the range from about 600 up to about 1200 C. The gaseous efiluent from the combustion zone is scrubbed to remove carbon dioxide and passed into an oxygen detector which measures oxygen depletion as a result of the combustion reaction. The technique thus generally described was primarily directed to the measuring of oxygen demands of waste streams. These systems contain relatively large amounts of combustible materials- Further development of the technique has subsequently demonstrated the capability of measuring the oxygen demand of relatively pure aqueous dispersions, provided certain modifications in technique are followed.

Accordingly, it is an object of the instant invention to provide a modified method for measuring the oxygen demand of aqueous dispersions containing small amounts of combustible materials.

Patented July 25, 1972 Particularly, it is an object to provide a technique for measuring the oxygen demand of systems wherein the demand is in the parts per billion range.

SUMMARY OF INVENTION The process of the instant invention comprises the steps of flowing a feed gas stream composed of an inert gas containing from about 0.5 up to 10 parts per million by volume of oxygen into a confined, heated combustion zone at a constant rate. Within the combustion zone is positioned a combustion supporting, porous catalyst bed at least about 8 centimeters long. The feed gas stream is passed through this catalyst bed and, upon emerging from the combustion zone, fed into a detector for small amounts of free oxygen.

Having established the feed gas stream, a small amount of an aqueous dispersion of a combustible material is injected into the gas stream within the combustion zone upstream from the catalyst bed. Preferably, it is projected or dropped onto the upstream end of the catalyst bed. The substantially oxygen-free aqueous sample is prepared by subjecting the aqueous sample to a vacuum or purging it with prepurified nitrogen, helium, argon or another inert gas. Regardless of the particular treatment utilized, the objective is to effectively reduce the amount of dissolved oxygen in the sample. This in turn maximizes the amount of oxygen extracted from the feed gas stream in the combustion reaction. However, if volatile oxygen demanding matter is present in the sample, some may be lost in the deoxygenation process. Therefore, if volatile matter is known to be present, low results may be expected.

By modifying the oxygen content of the feed gas stream, the length of the catalyst bed and utilizing aqueous samples which are substantially free of oxygen, the basic procedures as delineated in our co-pending application, Ser. No. 452,809, filed May 3, 1965, and now US. Pat. No. 3,560,156, is rendered capable of measuring oxygen demands of aqueous dispersions in the parts per billion range. The apparatus and other operational parameters of the invention are otherm'se as described in our co-pending application, the teachings of which in these particulars are incorporated herein by reference.

The operation of the instant invention will be better understood by reference to the accompanying drawing depicting the schematic layout of apparatus necessary to the practice of the instant invention. A complete description of this apparatus is contained in our co-pending application beginning at page 5 with line 20.

When operated in accordance with the instant invention, nitrogen from the inert gas supply tank 3 and oxygen from supply tank 4 are metered into the feed gas line 6 at relative rates so as to produce a gas mixture containing an amount of oxygen within the range from about 0.5 up to 10 parts per million by volume (S.T.P.) of oxygen. A suitable supply of oxygen can also be prepared by reducing the oxygen content of air to a suitable level. Quantitative measurement of oxygen at this level is difficult but an approximation of the desired oxygen level can be obtained by calibrating the oxygen detector 21 against specially purified nitrogen into which known quantities of oxygen are administered. A suitable pure nitrogen stream for this purpose is prepared by sparging commercial nitrogen, which often contains 50 parts per million by volume or more of oxygen, through pyrogallol or an aqueous solution of sodium sulfite activated by divalent cobalt ions.

The catalyst bed 16 is prepared from loosely packed platinum gauze balls. It is positioned within the combustion tube contained within the heating zone 13 of furnace 9. The bed is at least 8 centimeters long, preferably about 10 centimeters, and is placed so as to leave a sample expansion zone upstream from the catalyst bed. The gas flow rate is established'ata level within the range from about 50 to about 500 cubic centimeters (S.T.P.) and once a stable base line has been achieved on recorder 26, the system is ready for injection of the low oxygen demand sample.

A sample of the aqueous dispersion to be tested is suitably purged of oxygen for injection as by subjecting it to a vacuum of less than about 1 millimeter of mercury for 2 minutes or more. Alternatively, the sample may be purged with an inert gas.

In a particular operation, a gas cylinder was filled with a mixture containing parts per million by volume of oxygen. In etfect, the functions of the two supply tanks 3 and 4 of the drawing were combined within a single supply tank by prior dosing of the nitrogen with the desired known amount of oxygen. The feed gas stream and furnace were turned on. Within about 30 minutes,

the base line on the recorder had stabilized. A small aliquot of distilled water in the amount of about 20 microliters was then injected into the combustion zone upstream from the catalyst bed which was comprised of platinum gauze balls and constituted a porous bed approximately centimeters long within the combustion tube. The resulting decrease of oxygen content within the eflluent combustion products was determined and calibrated against a standard curve to yield an oxygen demand for the distilled water sample of 1100150 parts per billion by weight of oxygen equivalent relative to the combustibles in the water.

What is claimed is:

1. A process for measuring the oxygen demand of an aqueous dispersion containing a relatively low amount of combustible material whichcomprises the steps of (l) flowing a feed gas stream of an inert gas containing from about 0.5 up to about 10 parts per million by volume of oxygen at a constant rate through a confined combustion zone heated at a combustion supporting temperature within the range from about 600 to about 1200 C., and within the combustion zone, flowing the feed gas stream through a com- .4 bustion supporting catalyst bed at least about 8 centimeters long,

(2) flowing the effluent gas from the combustion zone into a continuous quantitative detector for gaseous oxygen to produce an electrical signal which varies with the oxygen content of the efiluent gas, and

(3) while continuing the above procedure, injecting a small amount of a substantially oxygen free aqueous dispersion of combustible material into the combustion zone upstream from the catalyst bed whereby an electrical signal correlative with the total oxygen demand of the aqueous dispersion is produced.

2. A method as in claim 1 wherein the aqueous dispersion of combustible material is rendered substantially free of oxygen by purging the liquid ,body from which the small sample aliquot was removed with an inert gas.

3. A method as in claim 1 wherein the aqueous dispersion of combustible material is rendered substantially free of oxygen by subjecting the liquid body from which the small aliquot was removed to a reduced pressure of at least 1 millimeter of mercury for at least 2 minutes.

4. A method as in claim 1 and including the additional step of measuring the amplitude of the maximum deviation of the signal produced by the oxygen detector in step 3 relative to the base line signal produced in step 2.

5. A method as in claim 4 and including the additional step of calibrating the measured amplitude of the maximum deviation.

References Cited UNITED STATES PATENTS 2,805,191 9/1957 Hersch 2041 T 3,205,045 9/ 1965 Lossberg 23230 PC 3,560,165 2/1971 Teal et al. 23230 PC 3,567,386 3/1971 Stenger 23-23O PC MORRIS O. WOLK, Primary Examiner R. E. SERWIN, Assistant Examiner US. Cl. X.R.

23-232 E, 253 PC, 254 E 

